The present invention pertains to microlithography (projection-transfer of a pattern, defined on a reticle or mask, onto a sensitive substrate using a charged particle beam (e.g., electron beam or ion beam) as an energy beam). Microlithography is a key technique used in the fabrication of microelectronic devices such as semiconductor integrated circuits, displays, or the like. More specifically, the invention pertains to methods for measuring and adjusting an illumination-optical system in a charged-particle-beam microlithography apparatus.
Conventional microlithography apparatus and methods are summarized below in the context of using an electron beam as a representative charged particle beam. Electron-beam microlithography has the potential of producing higher resolution than optical microlithography (using light such as ultraviolet light). Unfortunately, the throughput currently obtained using conventional microlithography apparatus is disappointingly low.
Various techniques have been evaluated to improve throughput. Exemplary current techniques include cell projection, in which certain regions of a pattern, especially regions in which a basic pattern unit is repeated a large number of times (e.g., memory cells), are transferred multiple times from the basic pattern unit as defined on the reticle. Cell projection is known also as a xe2x80x9cpattern-area batch-exposure system.xe2x80x9d When applied to mass production of semiconductor xe2x80x9cchips,xe2x80x9d however, throughput typically is an order of magnitude lower than realized using optical microlithography. As the feature sizes of integrated circuits continue to decrease with increasing miniaturization, throughput becomes increasingly critical for determining the economic viability of fabrication processes.
An approach developed to achieve substantially improved throughput of electron-beam microlithography is the so-called full-chip batch-transfer approach, in which the entire pattern as defined on the reticle is exposed in a single exposure or xe2x80x9cshot.xe2x80x9d The pattern is projected, with demagnification, onto the substrate using a projection lens. In other words, in this approach, all regions of the chip pattern on the reticle are illuminated at a single instant with an electron-beam xe2x80x9cillumination beam.xe2x80x9d Unfortunately, the larger the chip pattern, the greater the difficulty of adequately controlling aberrations over the entire optical field of the illumination-optical system and projection-optical system.
To solve the aberration problems experienced using the full-chip batch-transfer approach, a xe2x80x9cdivided-reticlexe2x80x9d approach has been proposed, in which the pattern as defined on the reticle is divided into multiple exposure units usually called xe2x80x9csubfields.xe2x80x9d The subfields, typically sized much larger than the pattern units used in cell projection, are exposed individually in a sequential manner onto a suitable substrate (e.g., semiconductor wafer). The respective images of the subfields as projected onto the substrate are placed so as to be xe2x80x9cstitchedxe2x80x9d together in a manner that yields a contiguous and properly aligned image of the entire pattern on the wafer. In this regard, reference is made to U.S. Pat. No. 5,260,151, incorporated herein by reference and Japan Kxc3x3kai Patent Document No. Hei 8-186070.
An electron beam as used for cell projection has a very small beam diameter (about 5 xcexcm at the reticle) for transferring the basic pattern units. Achieving homogeneous illumination of a region of such small dimensions on the reticle (i.e., achieving constant beam-current density across the illuminated region) is relatively easy to achieve. The electron beam is produced using an electron source having a conical electron-emitting surface. From the electron source, the illumination beam passes through a beam-shaping aperture that, especially because the beam is very narrow, does not degrade the transverse uniformity of the beam significantly. As a result, illumination homogeneity can be obtained.
However, with divided-reticle electron-beam microlithography apparatus, the area illuminated at any one instant by the illumination beam is much larger than in cell projection (e.g., subfield sizes of 1-mm square are used commonly). To illuminate such an area, a much larger-diameter illumination beam is used than used for cell-projection. Unfortunately, in conventional divided-reticle microlithography performed using a source having a conical electron-emitting surface, satisfactory uniformity of illumination is not achievable.
One approach to obtaining more uniform illumination in electron-beam microlithography apparatus (especially apparatus for performing batch transfer of large pattern areas) is using a source having a planar electron-emitting surface, and forming a xe2x80x9cfocusedxe2x80x9d image of the electron-emitting surface on the reticle. According to this approach, the quality of illumination reflects the quality and status of the beam source and of the lenses in the illumination-optical system. Unfortunately, using this approach, achieving actual illumination uniformity is exquisitely sensitive to variables such as (but not limited to) any of various operational parameters of the electron source and illumination-optical system, and variations in the planarity or condition of the electron-emission surface. Also, the various adjustments necessary to achieve and maintain uniform illumination uniformity are many and complex.
Highly accurate measurements of the transverse distribution of beam-current density (as a way to measure illumination uniformity) are required to make the adjustments to the electron-optical system and electron source necessary to achieve and maintain illumination uniformity. The measurement method conventionally used involves placing a measurement aperture (diameter of about 10 xcexcm, made by forming an aperture in a metal sheet, such as molybdenum or tantalum, that is about 1 mm thick) on the reticle stage or on the substrate stage (xe2x80x9cwafer stagexe2x80x9d). The illumination beam is scanned over the measurement aperture while measuring the current density of charged particles passing through the measurement aperture, thereby providing a measurement of the illumination distribution achieved by the illumination beam.
Forming a measurement aperture by etching a hole in a metal sheet is extremely difficult. Also, an aperture having a diameter of 10 xcexcm is essentially the smallest that can be made by etching a metal sheet. A 10-xcexcm diameter aperture simply is too large to achieve a sufficiently high measurement resolution necessary for current needs. Another problem with this conventional method is that the relatively thick metal sheet tends to absorb particles of the illumination beam and consequently is heated excessively during use for making measurements. The heating causes deformations of the sheet (and can cause actual melting of the sheet) during use. Because of these problems, the required fine adjustments of the beam source or of the lenses of the illumination-optical system to obtain a satisfactorily uniform illumination distribution cannot be performed using conventional methods or measurement devices.
In view of the shortcomings of conventional measurement devices and methods as summarized above, an object of the invention is to provide improved devices and methods for measuring and adjusting the distribution of current density of a charged-particle-beam (CPB) illumination beam passing through an illumination-optical system. Another object is to provide improved methods and devices for achieving uniformity (homogeneity) of the illumination beam as incident on the reticle.
To such ends and according to a first aspect of the invention, methods are provided for measuring the illumination uniformity of the illumination beam. The methods are provided in the context of performing CPB microlithography in which an illumination beam, generated by a planar CPB-emission surface and passing through an illumination-optical system, illuminates a region of a patterned reticle to produce an imaging beam directed by a projection-optical system to a substrate. In an embodiment of the subject method, an aperture plate is provided that defines a measurement aperture. The aperture plate is placed on the reticle stage at a reticle plane. The illumination beam is directed to be incident on the measurement aperture as the illumination beam is deflected across the measurement aperture in a scanning manner. Charged particles of the illumination beam passing through the measurement aperture are directed to a beam-current detector. Using the beam-current detector, the current-density profile of the charged particles incident on the detector is determined to obtain a measurement of the distribution of beam-current density of the illumination beam as incident on the measurement aperture. The aperture plate can be a silicon membrane (desirably having a thickness of no greater than 3 xcexcm), and the measurement aperture desirably is no greater than 2 xcexcm in diameter.
According to another aspect of the invention, methods are provided for adjusting an illumination uniformity of the illumination beam as incident on the reticle. The methods are provided in the same context as noted above, and involve a similar series of steps as the method summarized above. In addition, the subject adjustment method includes a step in which, based on the beam-current measurements obtained in the preceding step, at least one of the CPB source and the illumination-optical system is adjusted to obtain a homogeneous distribution of beam-current density as incident on the reticle.
According to another aspect of the invention, devices are provided (in the context of a CPB microlithography apparatus as summarized above) for measuring an illumination uniformity of the illumination beam as incident on the reticle. An embodiment of such a device comprises an aperture plate situated at a reticle plane on the reticle stage. The aperture plate defines a measurement aperture extending in a direction parallel to an optical axis of the CPB microlithography apparatus. The aperture plate desirably is a silicon membrane no more than 3 xcexcm thick, and the measurement aperture desirably is no greater than 2 xcexcm in diameter. The device also includes a beam-current detector situated at the substrate stage. The beam-current detector is configured to measure a current-density profile of the imaging beam as incident on the beam-current detector, so as to obtain a measurement of the distribution of beam-current density of the illumination beam as incident on the measurement aperture. The aperture plate can include a strut to strengthen and support the aperture plate.
Hence, uniform illumination intensity in a CPB microlithography apparatus configured to perform batch-transfer of a pattern from a divided reticle is obtained by using a CPB source having a planar CPB-emission surface. The illumination beam generated by the source is trimmed to the desired transverse profile by passage through a beam-shaping aperture. An image of the planar emission surface is focused at the beam-shaping aperture and at the reticle. As noted above, illumination uniformity is desired. However, conventionally, obtaining satisfactory illumination uniformity is extremely difficult because the necessary fine adjustments of the CPB source and/or of the illumination-optical system cannot be performed. The instant invention solves this problem by providing much more accurate and precise measurements of the distribution of beam-current density of the illumination beam. Based on information obtained from the measurements, fine adjustments to the CPB source and illumination-optical system now can be made as required to obtain the desired uniform intensity distribution.
In accordance with the invention, it has been discovered that silicon provides a suitable substrate material in which to define the tiny measurement aperture used in the subject devices and methods. Also, by making the aperture plate extremely thin (e.g., 1-3 xcexcm), the membrane scatters incident charged particles rather than absorbing them. Consequently, problems (e.g., aperture damage and/or dimensional changes) conventionally associated with heating of the material in which the aperture is formed are eliminated.